In a display system, one or more light sources are driven by a driving circuit for illuminating a display panel. For example, in a liquid crystal display (LCD) display system with light emitting diode (LED) backlight, an LED array is used for illuminating an LCD panel. An LED array usually comprises two or more LED strings, and each LED string comprises a group of LEDs connected in series. For each LED string, the forward voltage required to achieve a desired light output can vary with LED die sizes, LED die material, LED die lot variations, and temperature. Therefore, in order to generate desired light outputs with a uniform brightness, the forward voltage of each LED string should be adjusted such that the LED current flowing through each LED string is substantially the same. There are two traditional methods as shown in FIG. 1 and FIG. 2.
FIG. 1 shows a block diagram of a conventional LED driving circuit 100. The LED driving circuit 100 includes a DC/DC converter 102 for converting an input DC voltage Vin to a desired output DC voltage Vout for powering LED strings 108_1, 108_2, . . . 108_n. Each of the LED strings 108_1, 108_2, . . . 108_n is respectively coupled to a linear LED current regulator 106_1, 106_2, . . . 106_n in series. A selection circuit 104 receives monitoring signals from current sensing resistors Rsen_1, Rsen_2, . . . Rsen_n and generates a feedback signal. The DC/DC converter 102 adjusts the output DC voltage Vout based on the feedback signal. Operational amplifiers 110_1, 110_2, . . . 110_n in the linear LED current regulators compare a reference signal REF and the monitoring signals from current sensing resistors Rsen_1, Rsen_2, . . . Rsen_n respectively, and generate control signals to adjust the resistance of transistors Q1, Q2, . . . Qn respectively in a linear mode. In other words, the conventional LED driving circuit 100 controls transistors Q1, Q2, . . . Qn linearly to adjust the LED currents flowing through the LED strings 108_1, 108_2, . . . 108_n respectively. However, this solution may not be suitable for systems requiring relatively large LED current, which may result in a larger amount of heat generated by the transistors Q1, Q2, . . . Qn. As such, the power efficiency of the system may be decreased due to the heat/power dissipation.
FIG. 2 shows a block diagram of another conventional LED driving circuit 200. In FIG. 2, each LED string is coupled to a dedicated DC/DC converter 202_1, 202_2, . . . 202_n respectively. Each DC/DC converter 202_1, 202_2, . . . 202_n receives a feedback signal from a corresponding current sensing resistor Rsen_1, Rsen_2, . . . Rsen_n and adjusts an output voltage Vout_1, Vout_2, . . . Vout_n respectively according to a corresponding LED current demand. One of the drawbacks of this solution is that the system cost can be increased if there are a large number of LED strings, since a dedicated DC/DC converter is required for each LED string.